Vic Seasonal Temperature Outlook: probabilities for May to July 2007,
issued 23rd April 2007

Mixed seasonal temperature outlook

The national outlook for maximum temperatures averaged over May to July shows a
mixed pattern, with odds shifted towards above normal in the north and west, while
cooler than normal conditions are indicated in the far southeast of the mainland.

The pattern of seasonal temperature odds across Australia is due to recent higher
than average temperatures in tropical areas of both the Pacific and Indian Oceans,
with more influence coming from the Pacific.

Averaged over May to July, the chances are mainly between 60 and 70% for above-normal
maximum temperatures in the northern tropics and in western WA (see map).
So in years with ocean patterns like the current, about six or seven May to July
periods out of every ten are expected to be warmer than average in these
parts of Australia, with about two or three out of ten being cooler.

In contrast, the chances are between 35 and 40% for a warmer than average season
in southern Victoria and the southeast of SA. This means there's a 60 to 65% chance
of cooler than normal maximum temperatures averaged over May to July in these areas.

Outlook confidence is related to how consistently the Pacific and Indian
Oceans affect Australian temperatures. During the May to July period, history shows this
effect on maximum temperatures to be moderately consistent over large parts of the country,
but only weakly consistent in the southern half of Queensland (see background information).

Average seasonal minimum temperatures are favoured to be warmer than normal across
most of Australia's tropical regions, with the chances generally being between 60 and 70%.
In remaining areas the probabilities are in the neutral 45 to 60% range.

History shows the oceans' effect on minimum temperatures in the May to July period
to be moderately consistent over large parts of the country, with the exception of
SA and Victoria.

Click on the maps above for larger versions of the maps. Use the reload/refresh button to ensure the latest forecast maps are displayed.

More information on this outlook is available from
9.00am to 5.00pm (EST) Monday to Friday by
contacting the Climate and Consultancy section in
the Bureau's Victorian Regional Office: (03) 9669 4949.

Background Information

The Bureau's seasonal outlooks are general statements about the probability
or risk of wetter or drier than average weather over a three-month period.
The outlooks are based on the statistics of chance (the odds)
taken from Australian rainfall/temperatures
and sea surface temperature records for the tropical Pacific and Indian Oceans.
They are not, however, categorical predictions about future rainfall,
and they are not about rainfall within individual months
of the three-month outlook period.
The temperature outlooks are for the average maximum and minimum temperatures
for the entire three-month outlook period.
Information about whether individual days or weeks may be
unusually hot or cold, is unavailable.

Probability outlooks should not be used as if they were categorical forecasts.
More on probabilities is contained in the booklet
The Seasonal Climate Outlook - What it is and how to use it,
available from the National Climate Centre.
These outlooks should be used as a tool in risk management
and decision making.
The benefits accrue from long-term use, say over 10 years.
At any given time, the probabilities may seem inaccurate,
but taken over several years, the advantages of taking account
of the risks should outweigh the disadvantages.
For more information on the use of probabilities,
farmers could contact their local departments of agriculture or primary industry.

Model Consistency and Outlook Confidence:
Strong consistency means that tests of the model on historical
data show a high correlation between the most likely
outlook category (above/below median)
and the verifying observation (above/below median). In this
situation relatively high confidence can be placed in the
outlook probabilities.
Low consistency means the historical relationship, and
therefore outlook confidence, is weak.
In the places and seasons where the outlooks are most skilful,
the category of the eventual outcome (above or below median)
is consistent with the category favoured in the outlook
about 75% of the time.
In the least skilful areas,
the outlooks perform no better than random chance or guessing.
The rainfall outlooks perform best in eastern and
northern Australia between July and January,
but are less useful in autumn and in the west of the continent.
The skill at predicting seasonal maximum temperature peaks in
early winter and drops off marginally during the second half of the year.
The lowest point in skill occurs in early autumn.
The skill at predicting seasonal minimum temperature peaks in
late autumn and again in mid-spring.
There are also two distinct periods when the skill is lowest
- namely late summer and mid-winter.
However, it must always be remembered that the outlooks
are statements of chance or risk.
For example, if you were told there was a 50:50 chance
of a horse winning a race but it ran second,
the original assessment of a 50:50 chance could still have been correct.

The Southern Oscillation Index (SOI) is calculated using the
barometric pressure difference between Tahiti and Darwin.
The SOI is one indicator of the stage of El Niño or La Niña
events in the tropical Pacific Ocean.
It is best considered in conjunction with sea-surface temperatures,
which form the basis of the outlooks.
A moderate to strongly negative SOI (persistently below 10) is
usually characteristic of El Niño, which is often associated with below
average rainfall over eastern Australia, and a weaker than normal monsoon
in the north. A moderate to strongly positive SOI (persistently above +10)
is usually characteristic of La Niña, which is often associated with
above average rainfall over parts of tropical and eastern Australia,
and an earlier than normal start to the northern monsoon season.
The Australian impacts of 23 El Niño events since 1900 are summarized
on the Bureau's web site
(www.bom.gov.au/climate/enso/).